A self-sealing envelope has a main flap extension of the top edge of the front panel of the envelope and a secondary flap extension of the top edge of the back panel of the envelope, both flaps having an adhesive material (usually latex adhesive) applied to the same side, the main flap being foldable over the secondary flap so that the two layers of adhesive come into contact and seal the flaps together to close the envelope. Both flaps are foldable downwardly into an inoperative condition in which the adhesive surfaces lie face-to-face with surfaces of the envelope not having adhesive. To put this another way: whereas, in common with conventional envelopes, the main flap can be folded downwardly prior to the envelope being used and is prevented from adhering to the back panel of the envelope because the adhesive has not been moistened, in the case of a self-sealing envelope a secondary flap of the back panel of the envelope carries the complementary layer of adhesive and is also foldable downwardly so as to keep this complementary layer out of contact with the adhesive on the main flap until such time as it is desired to close the envelope, whereupon the secondary flap is raised to expose the adhesive layer.